Electrically controlled, vehicle, power assist, steering systems are well know in the art. In such systems, power assist typically varies as a function of applied steering torque and vehicle speed so as to maintain a desired steering feel. If vehicle speed is constant, an increase in applied steering torque results in an increase in power assist. If vehicle speed varies, power assist varies inversely in response thereto. Such systems are referred to in the art as "speed foldback control systems."
An example of an apparatus for controlling a power assist steering system is disclosed in U.S. Reissue Pat. No. Re 32,222 to Drutchas, which is assigned to the assignee of the present application. The Drutchas reissue patent discloses a steering system in which power assist is controlled in response to sensed steering torque and another input signal, such as sensed vehicle speed. A torque sensor provides an electrical signal indicative of applied steering torque. A steering wheel is connected to a rack member through a torsion bar. Upon application of steering torque, the torsion bar twists and a sensor, operatively coupled across the torsion bar, outputs a signal indicative of the amount of such twist. Taking into account the structural characteristics of the torsion bar, the amount of twist is indicative of the applied steering torque. The signal indicative of the applied steering torque and a signal indicative of sensed vehicle speed are connected to an electronic control unit. The electronic control unit controls power assist as a function of the sensed steering torque and sensed vehicle speed.
Another example of an apparatus for controlling a power steering system is disclosed in U.S. Pat. No. 4,538,698. The '698 patent discloses a steering system having an apparatus which controls power assist in response to applied steering torque and sensed vehicle speed. The apparatus includes a multiplier. The signal indicative of steering torque is connected to an input of the multiplier. A function generator connected to the speed sensor provides a signal to the multiplier having a value that is functionally related to vehicle speed. When the vehicle speed is within a particular speed range, power assist varies inversely in response to variations in the vehicle speed. The multiplier multiplies the value of the torque signal by the value of the signal output from the function generator. The output of the multiplier is coupled to a pulse-width modulating circuit through a differential amplifier. The output of the pulse-width modulating circuit is used to control a power assist motor.
Still another example of an apparatus for controlling a power assist steering system is disclosed in U.S. Pat. No. 4,562,896. The '896 patent is directed to an apparatus having an electrical circuit which controls power assist in response to vehicle speed. The circuit includes a speed sensor connected to a frequency/voltage ("F/V") converter. The output of the F/V converter is a DC voltage having a value that varies inversely in response to sensed vehicle speed. An error amplifying circuit compares the output voltage from the F/V converter against an electrical signal indicative of current flowing through a solenoid control valve and generates a corresponding error signal therefrom. The position of the solenoid valve is indicative of the amount of power assist. An oscillator circuit generates a triangular shaped, electrical signal at a reference frequency. The output of the error amplifying circuit and the output of the oscillator circuit are connected to a comparator. The output of the comparator is a pulse-width modulated signal having a duty cycle that varies as a function of vehicle speed The pulse-width modulated signal is connected to a solenoid drive circuit. The control arrangement is adapted so that power assist decreases as the vehicle speed increases.
Known speed foldback control systems for power assist steering systems, such as those shown in the '698 patent and the '896 patent, include complex circuitry. The complexity of the circuitry increases the overall number of electrical components which, in turn, adds to the total cost of manufacture and effects operating reliability of the circuitry. Furthermore, some known control circuits require use of a substantial number of trims, i.e., potentiometers, which must be adjusted during the manufacturing process. Due to the wide range of temperatures in which a vehicle must operate, stability in operation of the power assist control system is difficult when certain components, such as potentiometers, are used.